With the miniaturization of electronic devices, the electronic components used therein have also become progressively smaller and of higher performance, and the components are automatically mounted on printed circuit boards during assembly of the electronic devices. Surface-mounted electronic components are housed in carrier tape with pockets that are consecutively formed by thermoforming in accordance with the shapes of the electronic components. After loading the electronic components, a cover film is laid as a lid material over the top surface of the carrier tape, and the ends of the cover film are continuously heat-sealed in the longitudinal direction with a heated seal bar to form packages. As the cover film material, one having a heat seal layer of thermoplastic resin laminated onto a biaxially stretched polyester film substrate is used. The carrier tape is a thermoplastic resin, such as a polystyrene or a polycarbonate.
In recent years, electronic components such as capacitors, resistors, IC's, LED's, connectors and switching elements have become increasingly smaller, lighter and thinner, and it has become increasingly difficult to achieve the desired performance when peeling cover films to remove the electronic components contained inside from the packages. In particular, the peel strength when peeling the cover film from the carrier tape should continuously be within a certain range of values. The smaller and lighter the electronic components to be housed become, the more likely that the electronic components will fly out due to vibrations when peeling, resulting in problems during the mounting process.
In other words, when mounting components during the production of electronic devices and the like, the cover film is peeled from the carrier tape by an automatic peeling device, but if the peel strength is too strong, the cover film can be torn, and if too weak, the cover film may separate from the carrier tape during carrier transport, causing the electronic components inside to spill out. In particular, with the sudden increases in mounting speed, the peel speed of cover films has become very fast, with takt times of 0.1 seconds or less, and large impact stresses are applied to the cover film during peeling. As a result, the problem known as “film rupture” wherein the cover film is severed can occur.
As a measure against this kind of film rupture, a method of providing a layer of polypropylene, nylon or polyurethane excelling in impact resistance or tear propagation resistance between a substrate of a biaxially stretched polyester film or the like and a sealant layer has been proposed (see Patent Documents 1 to 3). Additionally, a method of preventing propagation of stress to the substrate layer by using a metallocene linear low-density polyethylene with a certain specific gravity as the intermediate layer to lower the Young's modulus of an adhesive layer between this intermediate layer and the substrate layer has been proposed (see Patent Document 4). However, even with these methods, it is difficult to suppress film ruptures during high-speed peeling such as 100 m per minute.
Additionally, packages for housing electronic components need to be baked in order to remove moisture contained in the sealing resins. In order to improve the productivity of such electronic components, the baking temperature must be raised to shorten the baking time. Recently, baking has usually been performed for 72 hours in a 60° C. environment or for 24 hours in a 80° C. environment with a cover film heat-sealed to a carrier tape. In such cases, the electronic components may adhere to the heat-seal surfaces of the cover film, causing mounting defects when mounting the components on a substrate. However, such problems of component adherence have not been sufficiently considered conventionally.